In general, servo valves for pressure systems exist in which a piston, under influence of a pressure difference between an intake or an outlet port and a control chamber, is movable between a closed and an open position. Due to use of the pressure difference, such valves are operable by way of relatively simple actuators having a low power consumption and weight, e.g. via a solenoid. The actuation may be arranged to move a pilot valve arranged to open or close a fluid passage between one of the ports and the control chamber. For various reasons, the existing valves of this kind are not suitable for vacuum systems, e.g. in connection with leak detection applications. In such systems, it is important to avoid any residual fluid in the valve between each operation thereof. Accordingly, valves and similar components of the system must support complete evacuation and due to a relatively complicated structural layout with various larger and smaller passages connecting the ports with the control chamber, drainage is typically a problem with the traditional servo valves. Moreover, the existing servo valves are made to operate across a certain pre-specified pressure difference. If a valve which is designed for pressure differences in the range of 1-100 bars is inserted across a pressure differential of less than one bar, the pressure difference would normally not be satisfactory for opening the valve, vice versa. Moreover, since the pressure difference disappears shortly after opening the valve, the servo effect of traditional servo valves would disappear. For this reason, traditional servo valves are not useful for vacuum systems.